tag:blogger.com,1999:blog-1812431336777691886.post3988812361687718691..comments2024-02-29T03:57:00.088-05:00Comments on The Mermaid's Tale: Skin color and vitamin D -- a beautiful theory destroyed by inconvenient facts? Anne Buchananhttp://www.blogger.com/profile/09212151396672651221noreply@blogger.comBlogger40125tag:blogger.com,1999:blog-1812431336777691886.post-27365984536850986482016-11-03T02:08:09.405-04:002016-11-03T02:08:09.405-04:00A recent study by Eaaswarkhanth and colleagues in ...A recent study by Eaaswarkhanth and colleagues in Genome Biology & Evolution (http://gbe.oxfordjournals.org/content/early/2016/09/26/gbe.evw242.abstract), didn't observe the claims of Thyssen et al that FLG loss-of-function is adaptive in northern latitudes with lower ultraviolet light exposure. In this new study, they have also shown that the frequency of FLG loss-of-function variants does not correlate with latitude attributing to the fact that the previous study suffered from ascertainment bias.Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-10883764223388957162016-11-03T01:49:11.016-04:002016-11-03T01:49:11.016-04:00This comment has been removed by the author.Anonymoushttps://www.blogger.com/profile/17111067841660067766noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-42033111654819425832015-03-31T08:44:48.039-04:002015-03-31T08:44:48.039-04:00The story must be complex because I think there is...The story must be complex because I think there is evidence of skin pigmentation variation more or less as we see it today, in north African rock art and in the Egyptian mummies and art, and the latter was around 6000 years ago. I don't know what the Egyptian's pigmentation genes show, but someone must. However, Egypt is a very sunny place, so why if the vit D or selection stories are correct, would they not have high levels of pigmentation? Maybe this is known but not to me, but it does complicate the idea that Europeans were still dark-skinned for a long time.Ken Weisshttps://www.blogger.com/profile/02049713123559138421noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-64820101216825016762015-03-31T06:48:46.485-04:002015-03-31T06:48:46.485-04:00The timeline for the lightening of the European sk...The timeline for the lightening of the European skin is now much better established: http://biorxiv.org/content/early/2015/03/13/016477. <br /><br />It seems that dark-skinned humans lived in Europe for tens of thousands of years (though the Neanderthals were light-skinned, even read-haired)Francesc Calafellhttp://biologiaevolutiva.org/fcalafellnoreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-82314807154747457272014-07-03T11:46:28.703-04:002014-07-03T11:46:28.703-04:00Thanks, Francesc. The plot definitely thickens. ...Thanks, Francesc. The plot definitely thickens. Anne Buchananhttps://www.blogger.com/profile/09212151396672651221noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-57413805543758105812014-07-03T11:43:58.434-04:002014-07-03T11:43:58.434-04:00I am unclear how this relates, as I think it perha...I am unclear how this relates, as I think it perhaps should, to the apparently lighter skin of Egyptians and northern Europeans at the first evidence (art and mummies, at least). <br /><br />This would not be the first TrueStory that wasn't so true, when it comes to evolutionary adaptive explanation. Personally, I think there is much that may be quite fundamental that we don't yet understand about selection.Ken Weisshttps://www.blogger.com/profile/02049713123559138421noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-16630678794209785852014-07-03T11:35:28.070-04:002014-07-03T11:35:28.070-04:00Thank you for these references!Thank you for these references!Holly Dunsworthhttps://www.blogger.com/profile/05260104967932801186noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-28753763415365316042014-07-03T11:18:21.173-04:002014-07-03T11:18:21.173-04:00There's another issue to consider in this stor...There's another issue to consider in this story: modern humans may have lived happily in northern latitudes for tens of thousands of years with dark skins. Two mesolithic ancient DNA samples, one from Sweden and one from Sapain, both about 8000 years old, carry the ancestral alleles at pigmentation genes, and may have had dark skins (Olalde et al. Nature. 2014 Mar 13;507(7491):225-8; Skoglund et al. Science. 2014 May 16;344(6185):747-50). The Spanish individual lived 100 miles and a 6,500-ft mountain range away from the see, so he may have consumed very little seafood. I hear that a third example is about to be published.<br />Intriguingly, ancient DNA samples from the Neolithic, just 2,000 years older, all carry the derived alleles in those loci, for which the genomic imprints of natural selection seem quite clear (Sturm, R. A. & Duffy, D. L. Human pigmentation genes under environmental selection. Genome Biol. 13, 248 (2012)). It may well be then that the selective pressure on these pigmentation genes was not exerted by latitude.Francesc Calafellhttp://bhusers.upf.edu/fcalafell/noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-57959892014418868462014-07-03T09:10:37.367-04:002014-07-03T09:10:37.367-04:00Why I had written about that example of a student&...Why I had written about that example of a student's hypothesis for body fur loss was *not* because that scenario isn't legitimate, and not because relaxed selection isn't a factor in how evolution unfolds but because of that very common non-understanding/awareness of it combined with unscientific and sloppy language. So many evolution students (a) assume that scenario *is* actually natural selection and write it when asked to provide a natural selection scenario for our body fur loss, and (b) don't know, and have a really hard time learning, that this scenario they provided is really about relaxed selection, mutation, and drift. Holly Dunsworthhttps://www.blogger.com/profile/05260104967932801186noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-48711847013795746052014-07-03T08:48:57.860-04:002014-07-03T08:48:57.860-04:00Unfortunately, even the Irish elk story has been q...Unfortunately, even the Irish elk story has been questioned. I don't know enough to have a view about that, except that to have gotten there in the first place, installed enough in the population for it to be found in the fossil record, it had to be 'fit' enough to replace its competitors. So we don't really know why it then became 'unfit' and it's just too easy, I think, to create stories that are hard to refute or to 'prove'.<br /><br />Your points in the first paragraph relate to widespread views about relaxed selection. They idea is that if a trait is not necessary enough, mutations will eventually damage or erase it. So, the view would go, if hair is covered by clothing and is no longer needed (for insulation) then mutations will make it eventually become very unorderly and then to go away. The energetic argument is that selection removes costly things because of tight darwinian competition. Of course how long this would take would depend on all sorts of things such as whether the same gene pathways are used in other indispensable traits, etc. The C-paradox, of the amount of total DNA relative to species phylogenies and relative to the amount that is 'functional' relates to the same idea, which is the belief that things that are costly must be detectable by selection. I think that's a mistaken idea and that complexity. All these points are involved in how people evaluate non-conserved aa's, which are often laden with blanket but untested or untestable assumptions. But I don't know enough about the specifics of filaggrin or skin biology to comment on that.Ken Weisshttps://www.blogger.com/profile/02049713123559138421noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-7728474163622641112014-07-03T08:34:37.667-04:002014-07-03T08:34:37.667-04:00I think one of the weakest statements in this pape...I think one of the weakest statements in this paper is the proposition that "the universal imperative for metabolic conservation" led to the demelanization of northern populations, because "they just didn't need to waste the energy making it anymore." That's the same low watt idea espoused by one of Holly's students, who attributed loss of body hair to the fact that since we wear clothes, we don't need hair any more.<br /><br /> This discussion isn't wanting for complexity, but consider also these facts. Filaggrin's role in supplying the stratum corneum with trans-urocanic acid is that of a barge full of histidine. "Loss of function" means it actually must be so mutated as to not show up at all - like vitellogenins, which are barges for hauling amino acids into eggs. Changes in non-conserved residues probably don't mean anything.<br /><br />Ken's comment is quite astute, that skin tone can be acted upon simultaneously by more than one selection pressure. There is the folate-protective role of melanin that was already pointed out. But there is also sexual selection to account for, and it does not stretch the imagination to think either or both sexes came to strongly prefer to mate with someone with light skin. Or almost no body hair. Or both. Sexual selection can sweep across a landscape of otherwise sensible physiology and push it into extremes that severely challenge the individual's survival. Think of those massive antlers on the Irish elk...Kirk Maxeyhttps://www.blogger.com/profile/11864529687578909475noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-32065598294753778812014-07-02T14:16:39.437-04:002014-07-02T14:16:39.437-04:00Those are just a few of the issues that make this ...Those are just a few of the issues that make this a difficult hypothesis to test. Anne Buchananhttps://www.blogger.com/profile/09212151396672651221noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-47884087102154880212014-07-02T13:20:37.120-04:002014-07-02T13:20:37.120-04:00Another nutritional/metabolic effect of melanin is...Another nutritional/metabolic effect of melanin is to prevent photodestruction of folic acid. Rev. Howard Furstnoreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-63903705418871102802014-07-02T12:27:25.226-04:002014-07-02T12:27:25.226-04:00The amount of exposure to ultraviolet light from t...The amount of exposure to ultraviolet light from the sun during the Pleistocene would have also depended on how much clothing they wore and also how frequently they went out doors. Muslim women who wear traditional Islamic clothing tend to have higher frequencies of rickets. <br /><br />Obviously, humans, especially Neandertals, living during the cold glacial periods of Europe would tend to stay indoors more and to cover much more of their body up with clothing while humans living in the tropics probably went around almost completely naked most of the time. Humans probably covered their infants up as much as possible to avoid exposure to the cold. <br /><br />Marcel Marcel F. Williamshttps://www.blogger.com/profile/16245086958213100840noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-10021316283546300412014-07-02T10:27:59.699-04:002014-07-02T10:27:59.699-04:00Thanks. This illustrates some of the issues, and ...Thanks. This illustrates some of the issues, and actually similar complexities regularly arise when simple evolutionary adaptive tales are told. Personally, I think biologists and anthropologists haven't got nearly enough respect for the challenges we face if we want to explain things that arose slowly, over long historical time periods, &c.Ken Weisshttps://www.blogger.com/profile/02049713123559138421noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-50498717662267212052014-07-02T10:03:41.373-04:002014-07-02T10:03:41.373-04:00I'm jumping into this late in the conversation...I'm jumping into this late in the conversation (and admittedly haven't read all of the comments), but Ken's comment --"What if 'skin color' is not a singular meaningful trait, but a complex part of some one or more systems, and hence affected by more than one, non-synchronous evolutionary history?"--seems logical to me. It's been a while since I did skin pigmentation research and I haven't kept up with the literature. But we know that there are skin pigments beyond eumelanin and pheomelanin (carotene and hemoglobin among them). And scientists have thought folate photolysis has played an important role in pigmentation change as well as vitamin D biosynthesis. Even the simple story of an inverse relationship between burning vs tanning responses isn't carefully considered (and may, in fact, be wrong). Given the differences of immediate and delayed tanning, I find it hard to believe that there isn't a great deal of information about immunological responses that have played roles in pigmentation evolution that we just don't understand yet (and I suspect it's likely that at least some pigmentation change was as by-products of other responses rather than any kind of direct adaptations to UVR). JKWhttps://www.blogger.com/profile/09292737413026824514noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-15542983136871865512014-07-02T09:49:33.106-04:002014-07-02T09:49:33.106-04:00Well various alternative terms would turn up many ...Well various alternative terms would turn up many thousands, and in a sense almost every human population relationships analysis is based on drift assumptions. But people typically assume, incorrectly, that this applies only to functionless sites in the genome, and that all the 'action' is driven by selection. Many or even most of my Crotchets columns have tried to address this (but the message doesn't sink in very well). Ken Weisshttps://www.blogger.com/profile/02049713123559138421noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-22911062076928182042014-07-02T09:34:38.313-04:002014-07-02T09:34:38.313-04:00Scholar google finds only 56 hits when I search fo...Scholar google finds only 56 hits when I search for human genetic drift in the title. Holly Dunsworthhttps://www.blogger.com/profile/05260104967932801186noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-22350673934501039792014-07-02T09:33:27.690-04:002014-07-02T09:33:27.690-04:00Well,the defense of that would be that we only inv...Well,the defense of that would be that we only invoke selection for such data after the fact when selection actually happened. When there wasn't any adaptation to explain, we don't.<br /><br />A general truth is that there is always enough variation to respond at least to gentle selective pressure (experiments show that), and drift and population dynamics leave particular signatures (for example, genetic distance patterns) that are found genomewide. The adaptive story would be expected to be stronger than the average demographic background. But unless strong and recent, selection's signature is very hard to detect against that background.Ken Weisshttps://www.blogger.com/profile/02049713123559138421noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-78546176463496044492014-07-02T09:30:07.569-04:002014-07-02T09:30:07.569-04:00Just thinking out loud...
When there's nice c...Just thinking out loud...<br /><br />When there's nice correlation between geography and a trait... why isn't the default assumption that drift and gene flow explain the cline? Adaptations require lucky mutations to pop up, and to get nearly just-as luckily propagated, and ALSO to affect survival and reproduction. That's a lot to expect and a lot to demonstrate with evidence too. Holly Dunsworthhttps://www.blogger.com/profile/05260104967932801186noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-28354649643047905982014-07-02T09:27:51.246-04:002014-07-02T09:27:51.246-04:00A good additional point! When someone says, hey! ...A good additional point! When someone says, hey! the Khoisan people are roughly as light as some Europeans and they get cooked by the sun every day. So the defensive posture for a hypothesis is that they didn't always live out there. In other words, a theory can always be defended by post hoc arguments. The problem, as you say about presentism, is that one can always do that but sometimes might be right--but usually we can't tell.Ken Weisshttps://www.blogger.com/profile/02049713123559138421noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-89843078840512068252014-07-02T09:22:30.950-04:002014-07-02T09:22:30.950-04:00And then there's the related problem of presen...And then there's the related problem of presentism. I've tried to explain to some friends that to expect every population's skin to match its UV environment runs against good evolutionary thinking, even if the melanin hypothesis is the correct and strongest explanation for human skin color variation. But I'm not sure I was able to get my view across. Holly Dunsworthhttps://www.blogger.com/profile/05260104967932801186noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-39635837654422067052014-07-02T09:21:49.851-04:002014-07-02T09:21:49.851-04:00I think they acknowledge the possibility of dietar...I think they acknowledge the possibility of dietary vitaD compensating with one hand and take it away with the other. It's definitely a potential confounder! But, the bigger problem is that such a small percentage of northern European populations have these FLG variants anyway, so the hypothesis, it seems to me, is already quite tenuous.Anne Buchananhttps://www.blogger.com/profile/09212151396672651221noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-2550611290093002412014-07-02T09:20:47.077-04:002014-07-02T09:20:47.077-04:00What if 'skin color' is not a singular mea...What if 'skin color' is not a singular meaningful trait, but a complex part of some one or more systems, and hence affected by more than one, non-synchronous evolutionary history?<br /><br />As noted in the post, melanin is involved in some immune system pathways (in ways I forget), so the global pattern could reflect differences in exposures, for example, and that could differentiate tropics from cold climates. This would make much of the other discussion more or less irrelevant, even if skin cancer and rickets and so on were also affected by pigmentation.Ken Weisshttps://www.blogger.com/profile/02049713123559138421noreply@blogger.comtag:blogger.com,1999:blog-1812431336777691886.post-61068630527837795312014-07-02T09:17:00.881-04:002014-07-02T09:17:00.881-04:00This. Is. Confusing!
As I see it, it only "b...This. Is. Confusing! <br />As I see it, it only "becomes more tenuous" if dietary VitD compensates for whatever's lacking (and shaping adaptations) in northern Europeans with relatively less/little dietary VitD.<br />Oh. I give up :) Holly Dunsworthhttps://www.blogger.com/profile/05260104967932801186noreply@blogger.com